1. Field of the Invention
The present invention relates to a spin tunnel transistor used for a high-density magnetic recording and reading magnetic head, etc.
2. Description of the Related Art
The recording density in the magnetic recording has been increased at an annual rate of 100 percent since the debut of GMR heads in which the giant magnetoresistive effect (GMR effect) is utilized. A GMR device includes a stacked film having the sandwich structure, Ferromagnetic metal layer/Non-magnetic metal layer/Ferromagnetic metal layer. Taking an example of such device, there is a spin-valve type magnetoresistive device, in which the magnetization of one of the ferromagnetic metal layers is fixed and the magnetization of the other ferromagnetic metal layer is changed according to an external magnetic field. The spin-valve type magnetoresistive device can detect a resistance value incorporating a change in the cosine of the relative angle of magnetization between the ferromagnetic metal layers.
Further, the development of a TMR device, in which the tunnel magnetoresistive effect (TMR effect) is utilized, is pursued in order to cope with higher density magnetic recording. A TMR device includes a stacked film of Ferromagnetic metal layer/Non-magnetic insulator/Ferromagnetic metal layer. In the TMR device, a voltage is applied between the ferromagnetic metal layers to detect the change in a relative angle of the magnetization between the two ferromagnetic metal layers as a change in tunnel resistance value.
In contrast, the development of three-terminal devices, referred to as spin tunnel transistors, is pursued. The device structures thereof include structure of: SMS (Semiconductor/Metal/Semiconductor) type; MIMS (Metal/Insulator/Metal/Semiconductor) type; and MIMIM (Metal/Insulator/Metal/Insulator/Metal) type. However, any of devices having the above structures are double-junction, three-terminal devices. In each of the double-junction, three-terminal devices, a base sandwiched between its emitter and collector is made of a metal (i.e. magnetic stacked film) (see U.S. Pat. No. 5,747,859 for MIMS structure).
The above-described spin tunnel transistor exhibits an extremely large change of magnetoresistance (MR) of several hundreds percentage, while the collector current is as remarkably small as 10−4 times the emitter current and therefore a ratio of collector current to emitter current (i.e. current transmittance) cannot increase. A remarkably small current transmittance is not suitable from the viewpoints of electric power consumption, working speeds, noises, etc.